Fiber optic communication systems are generally known. There are various types of fiber optic communication systems, including local area networks (LANs), wide area networks (WAN), and various other long-haul fiber optic communication systems. Various deleterious effects occur in fiber optic communication systems, such as, for example, the introduction of noise, optical loss, scattering effects, and wavelength dispersion. One of the last steps in the installation of a long-haul communication system is to configure the terminal receiver to receive the light transmitted over the link. Dispersion, however, complicates the set up of such a receiver. In a system employing a single channel, or a channel located at one wavelength, dispersion may be easily addressed during the deployment of the fiber and the repeaters and amplifiers in the system.
Most fiber optic systems, however, are configured to transmit tens or even hundreds of signals using wavelength division multiplexing (WDM). For example, systems have been designed that can carry 40 channels or 80 or more channels over the range from about 1450 nm to about 1650 nm or more using various amplifiers. Other wavelengths have also been employed such as those in the 1300 nm to 1400 nm range. These infrared wavelengths are used because these are the wavelengths at which traditional optical fibers (typically made using silica glass or various blends of silica glass) will transmit at a relatively low loss. Additionally, amplifiers such as erbium-doped fiber amplifiers and Raman amplifiers have been designed to amplify signals or channels at these wavelengths, thereby permitting long-haul communication of information with a low loss.
Because dispersion is a wavelength-dependent effect, compensation of dispersion in these systems is complex because some channels may require more compensation than others. Due to the dispersion slope of the fiber (i.e., the amount of dispersion as a function of wavelength), only one channel may be appropriately dispersion compensated in the deployment of the system. Therefore, the remainder of the channels may require dispersion compensation at the terminal receivers or optical amplifiers throughout the system. Because dispersion in a typical long-haul system may be as high as ±6000 ps/nm, failure to correct for dispersion may result in total loss of information on the signal.
The aforementioned dispersion compensation problem may be amplified by the increasingly complicated undersea systems. Such systems may include, for example, 16 fibers, each of which may have 32 of more WDM channels. Thus, at a terminal site as many as 512 or more channels may be received by the receiver, each of which may require dispersion compensation by various devices within the receiver. The conventional methods for compensating for dispersion are expensive and time-consuming and will be essentially obsolete with the increasing complexity of modern DWDM systems. It requires a considerable amount of labor to compensate each of the signals to provide a bit-error-rate (BER) in excess of 10−12.
One traditional method for compensating dispersion in an optical communication system includes splicing in long lengths of fiber that have a dispersion slope that is complementary (or otherwise opposite) to the dispersion slope of the original fiber. In order to conserve time, money, and man-power, in some systems, up to four channels may be compensated using a single span of fiber, thereby producing an acceptable, although imperfect compensation of the dispersion. This imperfect dispersion compensation is designed within the system signal-to-noise ratio (SNR) tolerances.
This method of dispersion compensation has a number of disadvantages. One such disadvantage is the introduction of loss to the signal such that additional optical amplifiers may need to be employed in order to compensate for the optical loss. This adds cost and complexity to the system, as amplifiers themselves are the source of system noise and may also account for additional deleterious effects in the system. Secondly, only the approximate accumulated dispersion is known beforehand, and thus a time-consuming trial and error approach may be required to determine the appropriate amount of dispersion compensation on a per-wavelength basis. This may lead to schedule delays and large financial penalties. Therefore, a quick, low-loss method may result in substantial financial savings.
The present invention may address the problems associated with dispersion in a timely manner and may be configured to simultaneously provide an agile wavelength selection filter. Further, the design can be tuned. In one exemplary embodiment of the present invention, the device may be tuned over, for example 10 nm or more. Thus, the entire C band (e.g., 1530 nm to about 1560 nm) can be covered with as little as 3 slightly different filter designs. For any given wavelength within the C band, an off the shelf dynamic receiver could be used in combination with the invention to demultiplex (DEMUX), dispersion compensate, and electronically convert the signal all within a matter of minutes. This technique may be lower loss and therefore, system gain requirements may be reduced.